Nonvolatile semiconductor memory device

ABSTRACT

A nonvolatile semiconductor memory device comprises a memory cell array in which memory cells each holding memory cell information are arrayed, reference cells which supply different reference currents respectively, and a read-out circuit. When reading the memory cell information from a selected one of the memory cells, the read-out circuit is brought into conduction to a first global bit line which is connected to a bit line of the selected memory cell, and brought into conduction to one of a plurality of second global bit lines respectively which are provided near the first global bit line and connected to bit lines of non-selected memory cells but not connected to the bit line of the selected memory cell, so that the memory cell information is determined by comparing a read-out current from the selected memory cell with each of the reference currents from the reference cells.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a U.S. continuation application which is filed under 35 USC 111(a) and claims the benefit under 35 USC 120 and 365(c) of International Application No. PCT/JP2003/02514, filed on Mar. 4, 2003, the entire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a reading operation of reading the memory cell information in a nonvolatile semiconductor memory device, and more particularly to a nonvolatile semiconductor memory device provided with a read-out circuit for reading the multi-level information held in the memory cells at high speed.

2. Description of the Related Art

In a nonvolatile semiconductor memory device, such as a flash memory, memory cell information is stored by using the current drive capacity of the nonvolatile transistor in the nonvolatile memory cell. That is, the memory cell information of “1” or “0” is stored with the nonvolatile transistor depending on whether the nonvolatile transistor passes the current or not, or depending on whether the nonvolatile transistor passes the increased current or the decreased current.

In recent years, the method of storing the 2-bit data in one memory cell wherein the potential held by one memory cell is divided into the four states: “1”, “2/3”, “1/3”, and “0” has been introduced in the nonvolatile semiconductor memory device, in order to raise the recording density.

For example, the information held by each memory cell is represented by the value of the 2 bits in which the data “11”, “10”, “01”, and “00” are assigned to the memory cell potentials “1”, “2/3”, “1/3”, and “0”, respectively.

FIG. 1 shows the composition of the read-out circuit of a conventional nonvolatile semiconductor memory device in which each memory cell holds the binary data.

The memory core portion 1 comprises the plurality of nonvolatile memory cells 10 (memory cell array) which are connected in the 2-dimensional matrix formation by the word lines WL and the bit lines BL, the decoder 11 to which the address signal is inputted, the column selector (YSEL) 12, the sector switch (SECY) 13, and the sector switch (SECY) 14.

In the nonvolatile semiconductor memory device, the read-out circuit (the sense circuit portion) compares the read-out current flowing on the global bit line or bit line connected to the selected memory cell, with the reference current from the reference cell to determine whether the read data is “1” or “0”.

In the nonvolatile semiconductor memory device of FIG. 1, each memory cell 10 stores and holds the binary data of “1” and “0”. At the time of read-out operation of the data, the current IGBL flowing on the global bit line GBL connected to the bit line BL of the memory cell 10 selected according to the address signal (Address) inputted is supplied to the cascode circuit (CASCOD) 31 via the data bus line LDB. The cascode circuit 31 converts the received data read-out current IGBL into the voltage signal, and outputs the voltage signal to the first input of the differential amplifier (SAMP) 30.

The reference cell (RC) 20 is provided outside the memory core portion 1, and it is a memory cell for supplying the reference current IREF to the differential amplifier 30. The reference current IREF which is supplied to the differential amplifier 30 by the reference cell 20 is set to the middle current value between the current flowing on the global bit line GBL when the data held by the memory cell 10 is “1” and the current flowing on the global bit line GBL when the data held by the memory cell 10 is “0”.

The reference current IREF from the reference cell (RC) 20 is supplied to the cascode circuit (CASCOD) 32 via the reference bus line to which the reference cell (RC) 20 is connected. The cascode circuit 32 converts the received reference current IREF into the voltage signal, and outputs the voltage signal to the second input of the differential amplifier 30.

At the differential amplifier 30, the judgment as to whether the read-out data of the memory cell 10 concerned is “1” or “0” is performed by comparing the voltage level of the memory cell current IGBL at the time of the data read-out with the voltage level of the reference current IREF.

In the nonvolatile semiconductor memory device of FIG. 1, the global bit line GBL is connected with the plurality of bit lines BL of the memory core portion 1 through the sector switch 13, and there is the hierarchical structure. The global bit line GBL is inputted into the column selector 12 for every predetermined number of bit lines, one of the bit lines is selected by the column selector 12, and the global bit line GBL is connected to the data bus line LDB by the selected bit line.

In the nonvolatile semiconductor memory device of FIG. 1, in order to correct the unbalance of the capacity of the parasitic elements in the sector switch 13 and the sector switch 14 on the side of the memory core portion 1, and in the column selector 12, the load capacitor CLD1 is additionally provided in the middle of the current path on the side of the reference cell 20, and the load capacitor CLD2 is additionally provided between the cascode circuit 31 on the side of the memory cell 10 and the differential amplifier 30.

On the other hand, in a case of the nonvolatile semiconductor memory device in which each memory cell holds the four-level data, for the purpose of judgment of the read-out data, the memory cell current at the time of the data read-out is compared with each of the three different reference-current values: the potential “3/4” which is the intermediate potential value of “1” and “2/3”, the potential “1/2” which is the intermediate potential value of “2/3” and “1/3”, and the potential “1/4” which is the intermediate potential value of “1/3” and “0”, respectively.

FIG. 2 shows the composition of the read-out circuit of a conventional nonvolatile semiconductor memory device in which each memory cell holds the four-level data. FIG. 3 is a waveform diagram for explaining operation of the data bus line at the time of read-out operation in the nonvolatile semiconductor memory device of FIG. 2.

Similar to the composition of FIG. 1, also in the nonvolatile semiconductor memory device of FIG. 2, the read-out circuit (sense circuit portion) compares the read-out current flowing on the global bit line GBL or the bit line BL connected to the selected memory cell 10, with the reference current from the reference cell to determine whether the read-out data is “1” or “0”.

As shown in FIG. 2, in the case in which the data held by each memory cell is the four-level data, one of the three different reference cells: the reference cell 21 (RC1) supplying the reference current corresponding to the intermediate potential “3/4”, the reference cell 22 (RC2) supplying the reference current corresponding to the intermediate potential “1/2”, and the reference cell 23 (RC3) supplying the reference current corresponding to the intermediate potential “1/4” is selected by the selector switch 24, and the read-out current is compared with the reference current from the selected reference cell. Then, it is determined whether the potential held by the memory cell concerned is any of “1”, “2/3”, “1/3”, and “0”.

Moreover, similar to the composition of FIG. 1, in order to correct the unbalance of the capacity of the parasitic elements, the load capacitor CLD2 is additionally provided between the cascode circuit 31 on the side of the memory cell 10 and the differential amplifier 30, and each of the load capacitors CLD11, CLD12 and CLD13 is additionally provided in the middle of the current path on the side of each of the reference cells 21, 22 and 23 respectively.

The above-mentioned reference cells 21-23 are arranged at external positions different from the memory cell array where the respective memory cells 10 are arranged, in order to avoid the stress of writing or erasing operation to the memory cells 10.

For this reason, in order to correct the parasitic resistance of the column selector 12 and the sector switches 13 and 14 existing on the side of the memory cell array, the junction capacitance of the sector switches 13 and 14 and the column selector 12, and the parasitic components with the adjoining non-selected bit lines BL and the global bit lines GBL, and the data bus lines LDB, the load capacitors CLD11, CLD12, and CLD13 are connected to the reference bus lines to which the reference cells 21, 22, and 23 are connected. By adding such load capacitors, the transient response characteristics of the reference bus line and the data bus line on the side of the memory cell are made equivalent, and shortening of the sense time of the differential amplifier 30 is achieved.

As described above, the conventional nonvolatile semiconductor memory device of FIG. 2 artificially makes the provision of the load capacitors CLD11, CLD12, and CLD13 in the current paths on the side of the reference cells in conformity with the capacity characteristics of the parasitic elements in the memory-cell-side current paths from the memory cells 10 to the differential amplifier 30.

However, the capacity of the parasitic elements in the current paths on the side of the memory cells varies with a certain width in the distribution due to the manufacturing tolerance. That is, the variation in the etching process of each wiring may cause the variation in the intervals of the adjoining wires, and the capacity of the parasitic elements with the intervals of the adjoining wires varies with a certain width in the distribution.

Moreover, the variation in the thickness of the interlayer insulation films may cause the variation in the intervals of the upper and lower layer wirings, and the capacity of the parasitic elements with the intervals of the adjoining wires varies with a certain width in the distribution. Furthermore, the junction capacitance and ON resistance may also vary with a certain width in the distribution by the variations of the parameters of the transistors, such as the gate oxide film, and the concentration and the depth of the diffusion layer.

In addition, the die size of the chip tends to be enlarged in connection with the progressing of larger capacity of the nonvolatile semiconductor memory device. Therefore, the difference of the on-chip parasitic capacitance in the same chip tends to be large. Simultaneously, the use of larger-diameter wafers also progresses and the difference within the wafer surface may also become large.

Strictly speaking, the current path formed from the memory cells arranged in the sector through the bit lines having the hierarchical structure, and the reference-current path directly linked from the reference cells arranged in the external region have the different physical parameters of the circumferential environment where the bit line is arranged, or the current path being formed.

For this reason, even if, in the design stage, the load capacitors CLD1, CLD2, and CLD3 which are suited to the characteristics of both the parasitic components are added, the variation of the manufacture and the tendency of the in-surface difference of the chip or wafer are different with each product, and it is difficult for the conventional nonvolatile semiconductor memory device to make the characteristics of the reference cells including these change factors in conformity with the characteristics of the parasitic components.

Moreover, FIG. 4A is a waveform diagram for explaining read-out operation of the conventional nonvolatile semiconductor memory device. As shown in FIG. 4A, in the conventional nonvolatile semiconductor memory device, the potential change in the current path (data bus voltage) on the side of the memory cells and the potential change in the current path (reference voltage) on the side of the reference cells in the transient state of the read-out operation are not in agreement in connection with the propagation delay of the signal by the parasitic components.

When the timing of detection of the memory cell information is moved in advance before the time the potential change is completed within the predetermined limits, there is the possibility that the memory cell information is incorrectly detected. To avoid the incorrect detection, it is necessary to delay the timing of detection of the memory cell information until the potential change is completed within the predetermined limits. This becomes the hindrance to making the read-out operation at higher speed.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an improved nonvolatile semiconductor memory device in which the above-described problems are eliminated.

Another object of the present invention is to provide a nonvolatile semiconductor memory device which is provided with a read-out circuit which enables high-speed reading operation of the memory cell information by eliminating the influences of the parasitic components in the current path on the side of the memory cells.

In order to achieve the above-mentioned objects, the present invention provides a nonvolatile semiconductor memory device comprising a memory cell array in which memory cells each holding memory cell information are arrayed, reference cells which supply different reference currents respectively, and a read-out circuit, wherein, when reading the memory cell information from a selected one of the memory cells, the read-out circuit is brought into conduction to a first global bit line which is connected to a bit line of the selected memory cell, and brought into conduction to one of a plurality of second global bit lines respectively which are provided near the first global bit line and connected to bit lines of non-selected memory cells but not connected to the bit line of the selected memory cell, so that the memory cell information is determined by comparing a read-out current from the selected memory cell with each of the reference currents from the reference cells.

According to the nonvolatile semiconductor memory device of the present invention, the reference cells which are equivalent to “3/4”, “1/2”, and “1/4” are respectively connected to three global bit lines which are provided near the global bit line connected to the bit line of the selected memory cell, and connected to the bit lines of only the non-selected memory cells. Each reference current and the read-out current of the selected memory cell are read out by setting the combination of the three global bit lines and the one global bit line connected to the bit line of the selected memory cell. The influence of the parasitic components in the current path on the side of the memory cells is eliminated, and high-speed reading operation of the memory cell information can be achieved.

Moreover, in order to achieve the above-mentioned objects, the present invention provides a nonvolatile semiconductor memory device comprising a memory cell array in which memory cells each holding memory cell information are arrayed, reference cells which supply different reference currents respectively, and a read-out circuit, wherein, when reading the memory cell information from a selected one of the memory cells, the read-out circuit is brought into conduction to a first bit line connected to the selected memory cell, and brought into conduction to one of a plurality of second bit lines respectively which are provided near the first bit line and connected to non-selected memory cells but not connected to the selected memory cell, so that the memory cell information is determined by comparing a read-out current from the selected memory cell with each of the reference currents from the reference cells.

According to the nonvolatile semiconductor memory device of the present invention, the reference cells which are equivalent to “3/4”, “1/2”, and “1/4” are respectively connected to three bit lines which are provided near the bit line connected to the selected memory cell, and connected to only the non-selected memory cells. Each reference current and the read-out current of the selected memory cell are read out by setting the combination of the three bit lines connected to the non-selected memory cells and the one bit line connected to the selected memory cell. The influence of the parasitic components in the current path on the side of the memory cells is eliminated, and high-speed reading operation of the memory cell information can be achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings.

FIG. 1 is a schematic diagram showing the read-out circuit of a conventional nonvolatile semiconductor memory device in which each memory cell holds the binary data.

FIG. 2 is a schematic diagram showing the read-out circuit of a conventional nonvolatile semiconductor memory device in which each memory cell holds the four-level data.

FIG. 3 is a waveform diagram for explaining operation of the data bus line at the time of read-out operation in the nonvolatile semiconductor memory device of FIG. 2.

FIG. 4A is a waveform diagram for explaining read-out operation of the conventional nonvolatile semiconductor memory device.

FIG. 4B is a waveform diagram for explaining read-out operation of the nonvolatile semiconductor memory device according to the invention.

FIG. 5 is a schematic diagram showing the read-out circuit of the nonvolatile semiconductor memory device in which each memory cell holds the four-level data in the first preferred embodiment of the invention.

FIG. 6 is a circuit diagram showing the load circuit in the read-out circuit of FIG. 5.

FIG. 7 is a circuit diagram showing the selector switch between the differential amplifier and the data bus line in the read-out circuit of FIG. 5.

FIG. 8 is a schematic diagram showing the read-out circuit of the nonvolatile semiconductor memory device in which each memory cell holds the four-level data in the second preferred embodiment of the invention.

FIG. 9 is a timing chart for explaining operation of the selector switch of FIG. 7.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A description will now be given of the preferred embodiments of the invention with reference to the accompanying drawings.

FIG. 5 shows the composition of the read-out circuit of the nonvolatile semiconductor memory device in the first preferred embodiment of the invention. In the nonvolatile semiconductor memory device of FIG. 5, each memory cell holds the four-level data, similar to the composition of FIG. 2.

The nonvolatile semiconductor memory device of FIG. 5 comprises the first memory core portion 1, the second memory core portion 1′ having the memory cell arrangement that is the same as that of the first memory core portion 1, and the read-out circuit.

The first memory core portion 1 comprises the plurality of nonvolatile memory cells 10 (first memory cell array) which are connected in the 2-dimensional matrix formation by the word lines WL and the bit lines BL, the decoder 11 to which the address signal is inputted, the column selector (YSEL) 12, the sector switch (SECY) 13, and the sector switch (SECY) 14 to which the address signal is inputted.

The second memory core portion 1′ comprises the plurality of nonvolatile memory cell 10′ (second memory cell array), the column selector 12′, the sector switch 13′, and the sector switch 14′.

In the nonvolatile semiconductor memory device of FIG. 5, the first and the second memory cell arrays 10, 10′, the column selectors 12, 12′, the sector switches 13, 13′, and the sector switches 14, 14′ have the three-layer hierarchical structure, respectively. The respective layers of each circuit component are designated by the suffixes 0, 1, and 2 attached to the reference character of the circuit component. For example, the first layers of the column selectors 12 and 12′ are designated by YSEL0 and YSEL0′.

As described above, the nonvolatile semiconductor memory device of FIG. 5 comprises the first memory cell array in which the plurality of first memory cells each holding the four-level memory cell information are arrayed and each first memory cell is provided to be selectable according to the address signal when reading the data, and the second memory cell array in which the plurality of second memory cells arrayed and each second memory cell is provided not to be selected according to the address signal when reading the data.

Moreover, in the nonvolatile semiconductor memory device of FIG. 5, the read-out circuit is provided to determine whether the read data is “1” or “0” by comparing the read-out current flowing on the global bit line or bit line connected to the selected memory cell 10, with the reference current from the reference cell.

The read-out circuit of FIG. 5 comprises the interface cascode circuit 51 connected with the column selector (YSEL0-2) 12 through the data bus line (LDB0-LDB2), the current-load cascode circuit 52 connected with the reference cell (not shown) through the reference bus line (LDB0′-LDB2′), and the differential amplifier (SAMP) 30 connected with the cascode circuit 51 and the cascode circuit 52.

Moreover, the read-out circuit of FIG. 5 comprises the switch 15 provided in the middle of the current path between the column selector 12 and the cascode circuit 51, and the switch 15′ provided in the middle of the current path between the column selector 12′ and the cascode circuit 31.

Furthermore, the read-out circuit of FIG. 5 comprises the load circuit (LD0-LD2) 16 connected with the current path from the column selector (YSEL0-2) 12, and the load circuit (LD0′-LD2′) 16′ connected with the current path from the column selector (YSEL0′-2′) 12′. The selection signal (RSW0′-RSW2′) is inputted to the load circuit (LD0-LD2) 16, and the selection signal (RSW0-RSW2) is inputted to the load circuit (LD0′-LD2′) 16.

In the read-out operation of the nonvolatile semiconductor memory device of FIG. 5, the word line WL is selected according to the inputted address signal, and the current according to the data which is held by the selected memory cell 10 flows on the bit line BL.

Any of the plurality of bit lines including this bit line BL are selected by the sector switches 13 and 14, and they are connected to one global bit line GBL0.

Furthermore, one of the three global bit lines GBL0-GBL2 including this global bit line GBL0 is selected by the column selector (YSEL0) 12 and brought into conduction to the data bus line LDB0. Simultaneously, the data bus line LDB0 is brought into conduction to the differential amplifier (SAMP) 30 by the switch 15. Although the data bus lines (LDB0-LDB2) are connected to the load circuits (LD0-LD2) 16 respectively, the conduction is cut away in each load circuit 16.

On the other hand, one of the three global bit lines GBL0′-GBL2′ with which the bit lines of only the non-selected memory cells 10′ are connected is selected by the column selector (YSEL0′) 12′, and brought into conduction to the data bus line LDB0′-LDB2′. The load circuits (LD0′-LD2′) 16′ are connected to the data bus lines (LDB0′-LDB2′) respectively.

In order to read the current flowing on the data bus line LDB0-LDB2, it is provided so that each reference current from the three reference cells which are equivalent to “1/4”, “1/2”, and “3/4” respectively flow on the data bus line LDB0′-LDB2′.

Moreover, similar to the data bus lines LDB0-LDB2, the data bus lines LDB0′-LDB2′ are connected to the differential amplifier 30 through the switch 15′.

The data bus lines LDB0′-LDB2′ are sequentially selected and brought into conduction to the differential amplifier 30 according to the switching selection operation of the switch 15′, and the differential amplifier 30 compares the read-out current from the data bus line LDB0 with each of the reference currents from the three reference cells to determine the read-out data of the selected memory cell 10.

On the other hand, when reading the data on the side of the data bus line LDB0′-LDB2′, the load circuit LD0-LD2 is connected with the data bus line LDB0-LDB2, and the similar read-out operation is performed according to the switching selection operation of the switch 15′.

FIG. 4B is a waveform diagram for explaining operation of the data bus line of the nonvolatile semiconductor memory device according to the invention.

As described above with FIG. 5, in the nonvolatile semiconductor memory device of this embodiment, the data bus line which is selected by the column selector 12′ from the three global bit lines, which are physically identical to the global bit line connected to the bit line of the selected memory cell 10, is conducted to each of the three reference cells which are equivalent to “3/4”, “1/2”, and “1/4” respectively, and the read-out operation is performed with the pair of the data bus line on the side of the selected memory cell 10 and the data bus line on the side of the non-selected memory cell 10′.

Since the junction capacitance of the sector switches 13′ and 14′ and the column selector 12′, and the parasitic capacitance with the adjoining non-selected wiring, etc. are contained in the three data bus lines, the data bus line on the side of the non-selected memory cell 10′ is essentially the same as the data bus line on the side of the memory cell 10 being read.

Since the transient change of the data bus line on the side of the memory cells in the read-out operation of the three reference bus lines is also the same, it is possible to move the timing of detection of the memory cell information in advance as shown in FIG. 4B.

Moreover, the neighboring global bit lines which are physically identical to the global bit line on the side of the first memory cell array are used, and when the influence of the noise etc. in the global bit line on the first memory cell array arises, the equivalent influence arises also in the global bit lines on the side of the second memory cell array, so that the influence of the first memory cell array and the influence of the second memory cell array are cancelled by each other.

Therefore, shortening the time from the beginning of the read-out operation to the end of the read-out operation is possible according to the nonvolatile semiconductor memory device of the present embodiment, and it is possible to shorten the period of the whole read-out operation.

FIG. 6 shows an example of the load circuit in the read-out circuit of FIG. 5.

As shown in FIG. 6, the bias voltage BIAS and the load voltage PLOAD are inputted to the reference cell circuit 54. In the reference cell circuit 54, for example, the reference current IREF0 which is equivalent to the memory cell potential “1/4” is passed from the reference cell RC0, and the reference current IREF0 is converted into the corresponding voltage signal.

The output voltage IBIAS0 is conducted to each of the load circuit (LD0) 16 and the load circuit (LD0′) 16′ which are connected to the data bus line LDB0 and the data bus line LDB0′ respectively, and the reference current IREF0 can be passed on the data bus lines LDB0 and LDB0′.

The load circuit (LD0) 16 comprises the transistor 16T1 to which the power supply voltage Vcc is inputted with the source terminal thereof being connected to the data bus line LDB0, the transistor 16T2 to which the power supply voltage Vcc is inputted, the transistor 16T3 to which the voltage IBIAS0 is inputted, and the transistor 16T4 to which the selection signal RSW0′ is inputted with the drain terminal thereof being grounded.

The load circuit (LD0′) 16′ comprises the transistor 16T1′ to which the power supply voltage Vcc is inputted with the source terminal thereof being connected to the data bus line LDB0′, the transistor 16T2′ to which the power supply voltage Vcc is inputted, the transistor 16T3′ to which the voltage IBIAS0 is inputted, and the transistor 16T4′ to which the selection signal RSW0 is inputted with the drain terminal thereof being grounded.

Similarly, the output voltage IBIAS1 from the reference cell circuit 54 containing the reference cell RC1 which supplies the reference current equivalent to the memory cell potential “1/2” is conducted to each of the load circuit (LD1) 16 and the load circuit (LD1′) 16′ which are connected to the data bus line LDB1 and the data bus line LDB1′ respectively, and the reference current IREF1 can be passed on the data bus lines LDBl and LDB1′.

Similarly, the output voltage IBIAS2 from the reference cell circuit 54 containing the reference cell RC2 which supplies the reference current equivalent to the memory cell potential “3/4” is conducted to each of the load circuit (LD2) 16 and the load circuit (LD2′) 16′ which are connected to the data bus line LDB2 and the data bus line LDB2′ respectively, and the reference current IREF2 can be passed on the data bus lines LDB2 and LDB2′.

With the load circuit 16 and the load circuit 16′ being provided as described above, the reference currents which are equivalent to the memory cell potentials “1/4”, “1/2”, and “3/4” can be passed on the data bus lines, respectively.

In the actual read-out operation in the nonvolatile semiconductor device of FIG. 5, when reading the current flowing on the data bus line LDB0-LDB2, the selection signal RSW0-RSW2 is set in ON state, the selection signal RSW0′-RSW2′ is set in OFF state, and each of the reference currents is passed on the data bus line LDB0′-LDB2′.

On the contrary, when reading the current flowing on the data bus line LDB0′-LDB2′, the selection signal RSW0′-RSW2′ is set in ON state, the selection signal RSW0-RSW2 is set in OFF state, and each of the reference currents is passed on the data bus line LDB0-LDB2.

FIG. 7 shows an example of the selector switch 15 between the differential amplifier 30 and the data bus line in the read-out circuit of FIG. 5.

The selector switch 15 of FIG. 7 comprises the transistor 15T0, the transistor 15T1, and the transistor 15T2. The signal RDSW0 is inputted to the gate terminal of the transistor 15T0, the source terminal thereof is connected to the data bus line LDB0, and the drain terminal thereof is connected to the input of the differential amplifier 30. The signal RDSW1 is inputted to the gate terminal of the transistor 15T1, the source terminal thereof is connected to the data bus line LDB1, and the drain terminal thereof is connected to the input of the differential amplifier 30. The signal RDSW2 is inputted to the gate terminal of the transistor 15T2, the source terminal thereof is connected to the data bus line LDB2, and the drain terminal thereof is connected to the input of the differential amplifier 30.

When the current flowing on the data bus line LDB0-LDB2 is read out, one of the selection signals RDSW0-RDSW2 inputted to the switch 15 is set in ON state. One of the data bus lines to which the three transistors 15T0-15T2 are respectively connected is selected according to this selection signal, and the selected data bus line is set in ON state, so that it is brought into conduction to the differential amplifier 30.

On the other hand, when the current flowing on the data bus line LDB0-LDB2 is used as the reference current, one of the selection signals RDSW0-RDSW2 inputted to the switch 15 is sequentially set in ON state. Each of the data bus lines to which the three transistors 15T0-15T2 are connected respectively is sequentially conducted to the differential amplifier 30 according to this selection signal, so that the read-out operation on the side of the data bus line LDB0′-LDB2′ is performed.

The composition and operation of the selector switch 15 have been explained with reference to FIG. 7. The selector switch 15′ in the read-out circuit of FIG. 5 can also be constituted in the same manner, and the selector switch 15′ can operate according to the selection signals RDSW0′-RDSW2′ which are inputted to the selector switch 15′, in the same manner as the selector switch 15 operates.

FIG. 9 is a timing chart for explaining operation of the selector switch of FIG. 7.

In the example of FIG. 9, the situation in which the current flowing on the data bus line LDB0-LDB2 is read out is assumed. The word line WL is selected in accordance with the inputted address signal (Address), and the current according to the data held by the selected record cell 10 flows on the bit line BL. The plurality of bit lines containing this bit line BL are selected by the sector switch SECY, and the selected bit lines are connected to the global bit line GBL0.

Furthermore, the three global bit lines GBL0-GBL2 including this global bit line GBL0 are selected by the column selector (YSEL0) 12, and they are brought into conduction to the data bus line LDB0. Simultaneously, the data bus line LDB0 is brought into conduction to the differential amplifier 30 by the selector switch 15. Although the data bus lines (LDB0-LDB2) are connected also to the load circuits (LD0-LD2) 16, the conduction is cut away in each of the load circuits 16.

On the other hand, the three global bit lines GBL0′-GBL2′ to which the bit lines of the non-selected memory cells 10′ are connected are selected by the column selector (YSEL0′) 12′, and they are brought into conduction to the data bus lines LDB0′-LDB2′. The load circuits (LD0′-LD2′) 16′ are connected to these data bus lines LDB0′-LDB2′. They are provided so that each of the reference currents from the three reference cells RC0, RC1, and RC2 which are equivalent to “1/4”, “1/2”, and “3/4”, flow on the data bus lines LDB0′-LDB2′ respectively.

At this time, each of the selection signals RDSW0-RDSW2 inputted to the selector switch 15′ is sequentially set in ON state as shown in FIG. 9. Each of the data bus lines LDB0′-LDB2′ is sequentially selected by the switching operation of the selector switch 15′ according to the selection signals RDSW0-RDSW2, and the selected data bus line is brought into conduction to the differential amplifier 30. Thus, the read-out operation on the side of the data bus lines LDB0-LDB2 is performed.

In the nonvolatile semiconductor memory device of the above-mentioned embodiment, the memory cells are connected to the plurality of bit lines, and the plurality of bit liens have the hierarchical structure such that some of the plurality of bit lines connected to the global bit line are selected by the sector switches 13 and 14.

However, the present invention is not limited to the composition of the above-mentioned embodiment. The nonvolatile semiconductor memory device of the present invention may be configured so that the bit lines to which the memory cells are connected are connected directly to the column selector 12, and the bit lines do not have the hierarchical structure. Even when such a composition is applied, the advantageous effects of the invention are the same as those of the above-mentioned embodiment.

FIG. 8 shows the composition of the read-out circuit of the nonvolatile semiconductor memory device in the second preferred embodiment of the invention. In the nonvolatile semiconductor memory device of FIG. 8, each memory cell holds the four-level data, similar to the composition of FIG. 5.

In the embodiment of FIG. 8, the bit lines BL to which the memory cells are connected are connected directly to the column selectors (YSEL0-2) 12 and the column selectors (YSEL0′-2′) 12′, and the bit lines BL do not have the hierarchical structure.

The composition of FIG. 8 is essentially the same as the composition of FIG. 5 except for the structure of the first memory core portion 1 and the second memory core portion 1′. In FIG. 8, the elements which are the same as corresponding elements in FIG. 5 are designated by the same reference numerals, and a description thereof will be omitted.

Even when the nonvolatile semiconductor memory device of FIG. 8 is applied, the advantageous effects which are the same as those of the composition of FIG. 5 can be acquired.

In the nonvolatile semiconductor memory device of FIG. 8, when reading the four-level data from the selected memory cell 10, each reference current and the current of the selected memory cell 10 are read out by setting the combination of the three bit lines connected to the non-selected memory cells 10′ and the one bit line connected to the selected memory cell 10.

As described in the foregoing, according to the nonvolatile semiconductor memory device of the present invention, in the reading operation, the combination of the global bit line connected to the selected memory cell, and the three global bit lines located near that global bit line and connected to the non-selected memory cells is set, the loads connected to the data bus lines are made ideally equivalent. The parasitic capacitance accompanied with the path gate which connects the global bit line and the data bus line is minimized. The memory cell information can be determined in the initial transient response period of the reading operation, and high-speed reading operation of the memory cell information can be achieved.

Moreover, the neighboring global bit lines which are physically identical to the global bit line on the side of the first memory cell array are used, and when the influence of the noise in the global bit line on the first memory cell array arises, the equivalent influence arises also in the global bit lines on the side of the second memory cell array, so that the influence of the first memory cell array and the influence of the second memory cell array are cancelled by each other. It is possible to raise the reliability of receding the memory cell information.

The present invention is not limited to the above-described embodiments, and variations and modifications may be made without departing from the scope of the present invention. 

1. A nonvolatile semiconductor memory device comprising a memory cell array in which memory cells each holding memory cell information are arrayed, reference cells which supply different reference currents respectively, and a read-out circuit, wherein, when reading the memory cell information from a selected one of the memory cells, the read-out circuit is brought into conduction to a first global bit line which is connected to a bit line of the selected memory cell, and brought into conduction to one of a plurality of second global bit lines respectively which are provided near the first global bit line and connected to bit lines of non-selected memory cells but not connected to the bit line of the selected memory cell, so that the memory cell information is determined by comparing a read-out current from the selected memory cell with each of the reference currents from the reference cells.
 2. The nonvolatile semiconductor memory device according to claim 1 wherein the memory cell array comprises a first memory cell array in which a plurality of first memory cells each holding a four-level data are arrayed and each first memory cell is provided to be selectable according to an address signal when reading the data, and a second memory cell array in which a plurality of second memory cells are arrayed and each second memory cell is provided not to be selected according to the address signal.
 3. The nonvolatile semiconductor memory device according to claim 1 wherein the reference cells comprise three reference cells which supply different reference currents respectively, and the plurality of second global bit lines comprise three global bit lines provided near the first global bit line.
 4. A nonvolatile semiconductor memory device comprising a memory cell array in which memory cells each holding memory cell information are arrayed, reference cells which supply different reference currents respectively, and a read-out circuit, wherein, when reading the memory cell information from a selected one of the memory cells, the read-out circuit is brought into conduction to a first bit line connected to the selected memory cell, and brought into conduction to one of a plurality of second bit lines respectively which are provided near the first bit line and connected to non-selected memory cells but not connected to the selected memory cell, so that the memory cell information is determined by comparing a read-out current from the selected memory cell with each of the reference currents from the reference cells.
 5. The nonvolatile semiconductor memory device according to claim 4 wherein the memory cell array comprises a first memory cell array in which a plurality of first memory cells each holding a four-level data are arrayed and each first memory cell is provided to be selectable according to an address signal when reading the data, and a second memory cell array in which a plurality of second memory cells are arrayed and each second memory cell is provided not to be selected according to the address signal.
 6. The nonvolatile semiconductor memory device according to claim 4 wherein the reference cells comprise three reference cells which supply different reference currents respectively, and the plurality of second bit lines comprise three bit lines provided near the first bit line. 